The influence of a PEDOT:PSS layer on the efficiency of a polymer light-emitting diode

The mutual influence of the constituent layers in a polymer light-emitting diode on the photoluminescence quantum efficiency of the light-emitting polymer is investigated. It is shown that a specific chemical interaction can occur between poly-(3,4-ethylenedioxythiophene):poly-(styrenesulphonic acid) (PEDOT:PSS) and poly-(p-phenylenevinylene) (PPV). It is further shown that PEDOT:PSS has a considerable effect on molecular dopants dispersed in the polymeric host. In the case of PPV the interaction with PEDOT:PSS results in the creation of defect states in an interface region between the PEDOT:PSS layer and the PPV layer. The presence of these defect states results in a considerable quenching of the PP-V photoluminescence. For devices based on PPV, already at voltages below the built-in voltage the PEDOT:PSS-induced defect states can be filled with charge carriers, a process that can be monitored with field-dependent photoluminescence measurements and with electrical impedance measurements. Filling of defect states with charge carriers constitutes a dynamic photoluminescence quenching mechanism, that is present in addition to the previously mentioned static quenching mechanism due to the presence of defect states. As a result, the photoluminescence intensity of PPV in a working device under operating conditions can be quenched by at least 20% with respect to the intensity at zero applied volt. To indicate that these effects involve a specific interaction between PEDOT:PSS and PPV, it is shown that the effects are absent in devices based on poly-(2,7-spirofluorene) (PSF) instead of PPV, and that the effects are independent of the type of cathode material. Defect states in PPV can be created through an electrophilic addition reaction involving the PPV vinylene bond and protons from PEDOT:PSS. (C) 2003 Elsevier B.V. All rights reserved.